Manufacture of printed circuits

ABSTRACT

Circuits including conductors and impedance elements are printed on paper. Component values are maintained within acceptable limits of variation by correlated selection of substrate porosity and ink to produce a high degree of absorption of the liquid ink vehicle into the porous substrate while producing relatively little penetration of the pigment. Resistance values are lowered, and their reliability and consistency greatly improved, as compared with printing on either impermeable or excessively porous substrates.

United States Patent Brand et al.

[451 Mar. 28, 1972 MANUFACTURE OF PRINTED CIRCUITS John Seemann Brand,Cranbury; Vladimir Paul Honeiser, Lawrenceville, both of NJ.

Assignee: American Can Company, New York, NY.

Filed: July 6, 1970 Appl. No.: 52,520

Inventors:

US. Cl ..ll7/2l2, 29/620, 117/216, 117/226, 1l7/DIG. 9, 174/685, 338/3081nt.Cl. ..B44d 1/18, H0111 37/36,1-l05h 3/12 Field oiSearch ..174/68.5;317/101 B, 101 C; 338/307, 308, 211, 212, 210; 29/620; 117/212, 217, 70R, 76 P, 226, 216, DIG. 9

References Cited UNITED STATES PATENTS 7/1944 Oldenboom 174 53,; 0);

Magdziarz .;.29/620 X Watson ..174/68.5 X

Primary Examiner-Darrell L. Clay Attorney-Robert P. Auber, George P.Ziehmer, Leonard R. Kohan and Leonard G. Nierman [5 7] ABSTRACT 7Claims, 6 Drawing Figures MANUFACTURE OF PRINTED CIRCUITS This inventionrelates to the manufacture of electrical components and circuits, andmore particularly to low-cost production thereof by use of high-speedprinting equipment.

The concept of employing a printing-press for mass production ofelectronic components and circuits is extremely old, having beenmentioned in numerous papers and books in the earliest days ofdevelopment of printed circuits." A few early experimenters apparentlyattempted to implement this idea, but were so unsuccessful that theefforts appear to have been abandoned as hopeless within a few yearsafter early widespread predictions that printing-press production ofelectronic circuits would shortly be commonplace. A number of earlypapers report experiments with printing presses attempting to printelectronic circuits, but usably reliable reproduction does not appear tohave been achieved. I

Paper is a wholly natural or obvious choice as a printing substrate, andappears to have been contemplated in the early speculative predictionsmentioned above. However, there have been found no reported experimentswith such a substrate. In view of the results of the investigationswhich have led to the present invention it may be speculated that theearly workers considered substrate porosity as such a handicap that thefailures with substrates such as plastics made it appear obvious thatresults'with porous substrates must be even more remote fromacceptability. It appears obvious (misleadingly, as shown by the presentinvention) that if difficulty is encountered in reasonably reproducing aresistance value (the most difficult problem as hereinafter discussed)on a highly stable and non-absorbing substrate such as a plastic plateor sheet, the difficulties in doing so on a porous substrate should becompletely prohibitive. The findings of the present inventiondemonstrate that this is not the case.

The early mentions in the literature discussed above appear to treat thedifficulties encountered in using a printing press as arising primarilyfrom limitations on the thickness of the ink layer which can bedeposited on a substrate surface. On this premise, it is indeedconcludable that best results from the standpoint of reproducibilityfrom one sample to the next should be obtained by deposition on a smoothsurface, rather than effectively losing a portion of the deposited filmin a porous substrate. However, as demonstrated by the presentinvention, the failure of earlier workers in the art to achievesatisfactory results with a printing press appears to have beenattributed to the wrong cause.

In the same early period of printed circuit development in which someworkers in the art unsuccessfully attempted to employ printing presses,they and others developed other graphic methods which came into commonuse for differing products. The product to which the term printedcircuit (PC) is now most commonly applied is neither printed" nor acircuit" within the meaning of these terms as hereinafter used. This isthe now-familiar type of circuit board limited to printed wiring forattachment of conventionally-manufactured impedance components. A secondmain line of development, in which impedance elements are also depositedas films by graphic-derived techniques, normally employs a glazedceramic or similar substrate upon which the films are deposited invarious manners which have become known in recent years as thick filmand thin film" deposition methods. The present invention is in essencean improvement in thick film" technology. (It will of course beunderstood that the term thick film as used in the art to contrast withthin film" does not alter the ordinary meaning of film as describing alayer which is in either case thin relative to the substrate.)

The invention is based on experimental investigation of the electricalcharacteristics of circuit components, particularly resistors,mass-produced by a printing press. The experimental data demonstratethat the production of satisfactory electrical circuits in this manneris feasible provided the inks and substrates employed are properlycorrelated in accordance with the criteria of selection which theinvention provides. Although it was the principal object of theexperimental work which resulted in the invention to make the use of aprinting press in such circuit production practical, the broader aspectsof the teachings may be employed to improve and simplify otherproduction methods, such as those employing silkscreening and similarmethods of deposition of films on substrates.

The satisfactoriness of any graphic method of film deposition for theproduction of electronic circuits requires the meeting of a large numberof requirements, some of which are met rather easily and some of whichare extremely difficult to meet. Irrespective of the process employed,there are three basic types ,of film compositions required, which may becharacterized as conductive, resistive, and dielectric. (The inventionmay be ultimately employed for the deposition of semiconductors, at suchtime as pigments of this type may be developed.) The conductive films orpatterns generally serve the same electrical function as wires andmetallic capacitor plates and coils in wired circuits. The primaryrequirement of such films is one of minimum conductivity. Although themeeting of this requirement is not simple from an absolute standpoint,the difficulty is relatively small as compared with the requirements inthe deposition of filmsin which variation from uniformity in eitherdirection is unacceptable. In the case of the dielectric films, normallyemployed between conductors to provide capacitance, close tolerancesmust be held to provide uniformity of capacitance value from one sampleto the next; also it is necessary to avoid pinhole imperfections, etc.,which can produce arcing. However, such problems are relatively easilysolved as compared with the problem of producing reliable resistancefilms.

The production of satisfactory resistance films has, at all times sincethe earliest days of development of graphic methods of circuitproduction, been a difficult problem. It would appear that thedifficulties encountered in this regard were a major factor in theapparent early abandonment of efforts to produce satisfactory componentcircuits with a printing press, and the shift of effort to inherentlyless economical processes, such as silk-screening of paints and vacuumdeposition of resistive metal films. Even with the complexities thusintroduced, presently known processes are in general incapable ofclosely reproducing resistance values without measurements made on eachspecimen. Much effort has long been devoted, and is still devoted, tocontrol of the resistance value of each individual specimen in responseto a measurement made after or during the film deposition, eitheremploying various techniques of trimming or fixing an end-point fordiscontinuance of a sputtering or similar process.

The present invention flows from observation that a printing-press has agreater inherent capability for producing wholly uniform film depositionthan other pattern-producing methods, followed by investigation of themanner in which this potential superiority can be realized in terms ofreliability of electrical characteristics of the deposited films. As aresult of experimental investigation of parameters producing a widerange of results as regards useability of the printed circuit output, ithas been ascertained that the assumption apparently heretofore made thatnon-porous materials are superior to porous materials as regards abilityto produce electrical uniformity of deposited films of inks and paintsis erroneous if the porosity characteristics of the substrate areproperly related to the composition of the ink or paint employed. Byvariation of porosity there are found to be produced with a given inkcomposition used in printing a wide variety of qualities of electricalcharacteristics of visually similar films, particularly as regardsreproducibility of resistance values, varying from products which areessentially useless to products of quality generally comparable to thatproduced by much more expensive methods of production. The employment ofeither an insufficiently porous or an excessively porous substrateproduces a wholly unsatisfactory product. Although the physicalphenomena which produce this great difference in useability of theprinted circuit product are not readily directly observable, availableexperimental evidence appears to demonstrate the theory of operationhereinafter to be discussed.

The type of film-pattern deposition typified by employment of a printingpress, silk screen, etc., employs a liquid or semiliquid composition ofthe type known variously as an ink, a paint or a paste, dependentprimarily on the general degree of viscosity or plasticity. Such amedium in general consists of particles of solid material, herein calledthe pigment, along with liquid components in which the particulatematter is dispersed to give the overall composition the liquid orsemiliquid character required for the process employed and to provide anultimate binder for the particles and adherence to the substrate. Afterdeposition of the film, whether by spraying, brushing, printing-pressoperation, or otherwise, the liquid components are partially solidifiedand partially eliminated by evaporation, etc., with the solidifiedportion forming a binder or matrix holding the pigment particles andadhering to the substrate. As a practical matter, most compositions forforming such films have substantially more than two components. Ink andpaint compositions commonly employ separate constituents for the bindermaterial and a diluent employed to impart the desired viscosityproperties, and there are also present additives to speed the drying orcuring and for similar purposes.

It is theoretically possible to employ only sufficient liquid in such acomposition to form, when solidified, a binder for the pigment of theminimum concentration which will hold it together. However, the inkcompositions employed in printing presses desirably contain aconsiderable component of liquid beyond this minimum. Further, even weresuch an ideal to be achieved, there is doubt whether it would bepractically useful on a fluid-impervious smooth substrate, since theconcentration of binder required to give assured bonding to a smoothimpermeable substrate is prone to be substantially higher than thatwhich is required for mere cohesiveness of the dried film.

Where a film is produced on an impermeable substrate from a liquidcomposition having a much higher concentration of binder than theminimum required for cohesiveness of the ultimate film, electricalperformance is found to suffer wide variations in successive samples.Consider, for example, the conductivity of a film of metallic pigmentparticles. A theoretically optimum conducting film so deposited wouldhave no more binder than that required to fill the interstices or voidsinherent in a packed-particle structure. Current flow distribution insuch a structure is generally similar to that through a wire ofcomparable metal content. But with even mildly excessive binder, contactbetween adjacent particles is no longer assured, and successive shortlengths of a conductor so formed can exhibit wide variations inconductivity. The current pattern through such a conductor is whollydifferent than that through a wire, the overall conductivity of such aconductor being primarily determined by randomly-occurring regions oflow conductivity. Nevertheless, because the primary performancerequirement of a conductor is some minimum of conductivity, i.e., amaximum resistance, a small excess of binder can be tolerated, althoughnot desirable.

ln the case ofa resistor, the adverse effects of excess binder in thedried film are much more prohibitive. Random variations in resistancefrom sample to sample will occur even with an ideally uniform depositionproducing wholly identical patterns and volumes of deposition ofidentical composition. It is believed a practical impossibility todeposit a film, particularly with a printing-press, having so littleexcess of binder present in the composition as deposited that reliableresistances can be achieved.

In the method of the present invention, the film is deposited withsubstantial excess liquid vehicle and the excess vehicle is removed fromthe film prior to solidification by a filtering action which leaves thepigment in a relatively low concentration of binder material on thesurface of the substrate to closely approximate the idealizedfilm-production already discussed. The ink is deposited with an excessof binder on a porous substrate surface having a pore-size selected toact as a filter passing the excess binder while admitting relativelylittle ofthe pigment from the surface of the substrate. The liquid ofthe ink composition is removed to the point where the film, upon drying,contains an amount of binder closely approximating the theoreticaloptimum earlier described. The constancy of resistance values from onecircuit to the next obtained in the printing-press production ofcircuits compares favorably with that obtained by far more complex andexpensive production methods.

The pore-structure for producing the filtering action is selected byexperimental matching to the characteristics of the particular inkcomposition, particularly in accordance with the size of the particlesof the resistive pigment (normally carbon). The most readily availableand inexpensive type of porous substrate material is of course paper. Itis found, however, that most ordinary papers are not suitable with thepigments most practical for use. As in the case of an ordinary discretecarbon resistor, the employment of relatively coarse carbon particlesproduces resistors of prohibitively high noisegeneration, as well asmaking reproducibility of resistance values difiicult or impossible. Itis found that papers selected without care, when employed with pigmentsof relatively fine particle-size giving the best resistor performance,produce wholly insufficient filtering action. Where the pore structureis so coarse as to permit fairly free penetration of the pigmentparticles, the ink is absorbed in the paper with little or no filteringaction, and the results are at least as unacceptable as in the case ofimpervious substrates. With proper pore structure at the surface of thesubstrate, the pigment is retained in the surface while the excessbinder is absorbed into the body of the substrate. Upon solidificationof the binder, there remains on the surface a matrix containing thepigment particles with approximately the minimum amount of binderrequired to provide cohesiveness. This is securely bound to thesubstrate by the absorbed binder material, which contains relativelylittle pigment. The concentration of binder in the surface film can bewell below that required to obtain adherence to a nonporous substrate.

lt will of course be understood that neither the pores of the substratenor the pigment particles are in practice wholly uniform in size, sothat there is no abrupt interface between pigmented and unpigmentedbinder at the surface of the substrate. However the volume proportion ofpigment to binder in the pores of the ultimate product is substantiallyless than the volume proportion of pigment to binder in the surfacefilm, the pigment concentration decreasing very rapidly with depth ofpenetration of the binder.

Various types of porous substrates may be employed successfully. Themost desirable arepapers having a small-pored coating on the surfaceupon which the circuit is printed, the fineness of pores required beingdifficult to obtain in the body of a paper. Among commercially availablepapers, clay-coated papers appear most suitable for use with thefine-ground carbon pigments which are preferred for resistivecomponents.

The invention further provides certain improvements or refinements ofthe general process as above described. Practical printing of electroniccircuits requires a plurality of successive layers or superimposedfilms. Conductor patterns are coupled to margins of resistive patternsand capacitors are desirably formed by a number of successive layers ofdeposition. Layers above the first layer cannot employ the filteringaction described above. This is found to constitute no major restrictionon the utility of the invention, since it is only the resistanceelements in which the filtering action is found highly critical toacceptability. Accordingly, the over-printing of successive layers ofnon-resistive ink patterns may, if so desired, be performed with inks ofthe same composition as the base layer. For best results it is founddesirable to employ, for the deposition of overprinted patterns, inkswhich permit substantial removal of liquid by means other than fluidflow. One manner of accomplishing this is by employment of a combustibleliquid as a primary constituent, deposition of the film being followedby flaming or flashing. Such a printing process is greatly advantageousin that it providesvery rapid curing of the binder constituent andoverall drying of the film. However, as hereinafter discussed morefully, it is frequently desirable to employ for resistive films, whereprecision is required, a type of ink which dries more slowly but givessomewhat better compaction of the pigment particles as a result of thefiltering action of the substrate.

As will hereinafter be seen, additional factors enter into the selectionof inks and substrates and other aspects of the present method, such asassuring that the desired degree of removal of liquid from the depositedfilm occurs prior to solidification of the binder.

In addition to the aspects above briefly described, the invention alsoprovides further features best understood from the description below,illustrated in the annexed drawing.

In the drawing:

FIG. 1 is a schematic enlarged fragmentary sectional view of a printedcircuit element made in accordance with the invention;

FIG. 2 is a schematic illustration of the random-contact orientation ofpigment particles when incorporated in a substantial excess of binder;

FIG. 3 is a schematic diagram corresponding to FIG. 2 but illustrating amore ideal relation of pigment particles;

FIG. 4 is an idealized graph illustrating the general approximaterelation between resistance and substrate pore-size for film patternsformed by identical deposition of a given printing ink;

FIG. 5 is a schematic illustration of the operation of a rotaryletterpress printing press employed in the printing of circuits inaccordance withthe invention; and

FIG. -6 is a fragmentary schematic sectional view of a capacitor formedby overprinting a number of layers in accordance with the invention.

Referring first to FIG. 1, there is there illustrated in schematic forma greatly magnified section of a printed circuit element of theinvention comprising a resistance element film pattern 10 on a substrategenerally indicated at 12. Appropriate film pattern shapes for formationof circuit elements are well-known, for example as shown in US. Pat. No.3,484,654 of Vladimir Paul I-Ioneiser, and accordingly not illustrated.The substrate is here a coated paper, the body 14 being coated with asurface layer of a filter coating 16, such as the clay coating ofcertain commercially available papers later mentioned. Although acoating on only one side is illustrated, it will of course be understoodthat coatings on both sides are employed where circuit patterns areprinted on both sides of a substrate.

As schematically shown in FIG. 2, where a film pattern is formed whichhas a substantially higher proportion of binder than the minimumrequired to hold the pigment particles 18 in a cohesive matrix, contactsbetween adjacent particles 18 occur in a random manner to form distinctpaths for current flow (the direction of current flow is arbitrarilyselected as left to right in the illustration of FIG. 2, with arrowsrepresenting exemplary complete current paths). Much of the particulatecontent contributes nothing to the conductivity. The resistance valuethrough such a structure cannot be expected to be closely reproducedfrom one sample to the next, irrespective of the precision with whichdeposition conditions are duplicated in each successive specimen. Bycontrast, where the packing of the particles more closely approaches theideal, as in the particles 20 of FIG. 3, the fluctuations of resistancevalue from sample to sample may be expected to be much lower. inaddition to the fact that the resistance itself will be much lower. Inaccordance with the invention, conditions closely approaching those ofFIG. 3 are obtained by removing from the film, after its deposition butbefore completion of drying or curing, substantially all of the excessbinder (shown as voids in FIGS. 2 and 3).

The expected general or gross effect of varying the porositycharacteristics of substrates upon resistance values of a resistivefilm, with deposit of a given quantity of the same ink in apredetermined pattern, is shown in the graph of FIG. 4. As there shown,the porosity or pore-size characteristics may be considered to havethree general regions. The lowermost region is designated that ofimpermeability and has as its lower limit a substrate which is whollyimpermeable, such as a solid sheet of smooth plastic, a paper with acoating of solid plastic, or a glazed ceramic. As a very small degree ofporosity is introduced and then increased, the resistance of the driedfilm decreases due to binder absorption prior to curing, until there isreached the porosity region producing a high degree of filtration. Inthis region essentially all of the excess liquid over that required forbinding of the film is absorbed in the substrate so that uponsolidification of the binder the pigment in the film approximates theideal packed condition. When the pore size becomes too coarse, asubstantial portion of the pigment itself penetrates into the paper,wherein it is lost as a factor of conductivity, to a degree which cannotbe exactly controlled, this region demonstrating a rise of resistancedue to such pigment absorption. In principle, if the porosity were to beincreased to the point where the substrate is effectively merely a veryloose fibrous mass with substantially no isolation of the regions intowhich the liquid flows, there would be a further region (not shown) inwhich the resistance would again approach the value on a non-poroussurface. However, such a substrate cannot preserve the shape of theoriginal deposited pattern with sufficient precision to be of utility.

The existence of the three general regions shown in FIG. 4 has beenverified experimentally, but there are found to be too many practicalvariables to permit the obtaining of experimental data closelydelineating a smooth idealized curve such as shown. No method ofdeposition is presently known for exactly reproducing the amount of inkdeposited on substantially differing substrates while at the same timemaintaining constant all of the other variables which may affect theresult. For example, although a printing press, as later mentioned, isfully capable of leaving substantially identical quantities of an ink onextremely large numbers of successive substrates of the samecompositions, a substantial change of surface composition greatlyaffects the amount of ink deposited in each impression. Although suchfactors as impression pressure may be adjusted and correlated withobserved ink consumption to equalize the amount of ink deposition onvarying substrates, such adjustments in themselves represent changes ofconditions which can materially affect the data. In addition, porousmaterials are not sufficiently calibratable in exact pore size to permitconstruction of a continuous curve such as that illustrated fromexperimental data. However, the validity of the general shape of theplot of FIG. 4 appears to be demonstrated by the experimental evidencehereinafter described, obtained in experiments with substrates fallingin the two high-resistance regions as well as a number of clay-coatedpaper substrates appearing to possess varying degrees of close approachto optimum porosity characteristics with the inks employed.

There is shown in FIG. 5 a schematic illustration of the rotaryletterpress printing process. Although the invention in its broadaspects may be employed with printing processes and equipment of othertypes, these presently appear to be less satisfactory. It is found thatthe reproduction of resistance values is a far more sensitive indicatorof uniformity of deposited films than any other known, and that bestresults are obtained with the rotary letterpress process.

The illustrated rotary letterpress will be recognized as conventional bythose skilled in the printing arts. The plate cylinder 22 and theimpression cylinder 24 rotate at constant speed to deposit on thesubstrate, on the plate pattern, the ink delivered to the patternedplate by the train of inking rollers 26 from a suitable ink reservoirand fountain rolls. The constant-speed rotary motion of all componentsproduces a uniformity of film deposition from sample to sample whichcannot be achieved in any other known manner. However, for the basicreasons already outlined, assuring constancy of film deposition fromsample to sample has been found to be wholly insufficient in itself toproduce corresponding uniformity of the electrical characteristics ofcircuits so printed. Despite the precision of repetition of depositedink films inherent in such a press, no fully satisfactory results arefound to be obtained with either impervious substrate materials or withpapers not giving the filtering action earlier described.

The commercially available papers found most desirable are papers havinga coated surface of much finer pore structure than the body or core.Particularly advantageous are papers having a surface layer offine-ground particles, such as claycoated papers. Even among papers ofthis description there are appreciable differences in merit as measuredby variation of resistance values among a large number of samplesprinted on each particular paper. However these differences in merit (asmeasured by the statistical variation of individual samples about themean resistance value obtained with the particular paper) are relativelyminor as compared with the superiority of any of such coated papers overeither uncoated papers, on the one hand, or impermeable substrates, onthe other.

Exact comparison of the merit of papers having only small differences instructure or composition is difficult. There are various factors, laterdiscussed, which enter into selection of particular papers for use withany given ink. Also there are a number of variables which make closequantitative comparison between reproducibility results achieved in runswith respective papers extremely difficult when they are in the centralor optimal region of porosity of FIG. 4. Were the ideal single-variablecomparison of FIG. 4, with all else held constant, practically possibleto measure, mere mean resistance value would be an inverse indicator, ofreproducibility merit. As hereinafter seen, however, the variables ofthe printing process cannot be controlled sufficiently accurately topermit the obtaining of detailed data points clearly demonstrating thiscorrelation among papers of closely comparable porosity.

The discussion thus far is simplistic in describing the requirementswhich must be met in practical printing of circuit patterns,particularly resistance patterns, whether with a printing press or byother means for deposition of the films. At

least equally important with the preservation of resistance value of agiven resistor of a circuit from one sample to the next is preservationof a constant ratio of resistance values. Where deviations from the meanvalue occur on a random statistical basis, the deleterious effects oncircuit performance are in essence multiplicative in many circuits. Thepreservation of wholly exact identity of such printing conditions aspressure at the point of impression, etc., over a pattern ofsubstantialsize is found to be difficult, even with presses of precisionsufficiently high so that there existed, prior to the present invention,no manner of detecting nonuniformities except by an instrument such asan optical densitometer. It is accordingly desirable to select papersand inks so that the effects of any such variations which occur will beminimized. Thus a selection of parameters which produces the bestresults from the standpoint of identity of successive samples of asingle resistor is not necessarily an optimum selection for the printingofan overall circuit.

Experiments were performed with letterpress printing on high-gradecommercial rotary presses of two different manufacturers (Davidson andHeidelberg). The similarity of results indicated that differences indetail of press construction do not have any major effect on productquality, so long as the full precision of reproduction of which theletterpress process is capable is closely approached. Experiments withother printing processes, namely offset and flexographic, indicatedthese to be less satisfactory.

The experiments which resulted in the invention employed a substantialnumber ofink formulations (for each ofthe three general types of films)on a large variety of paper substrates, ranging from wholly impermeableplastic-coated papers to fairly coarse uncoated papers. Papers at theseextremes of the porosity range were found incapable of producingsatisfactory circuits with any type of ink composition. Althoughpassable results could be obtained with a few uncoated papers as regurdsconductive films, by selection of pigment having a large effective sizefor purposes of producing the filtering action, and although theconductive films thus deposited can, with certain precautions, beoverprinted with dielectric films and a further conductive film to formfairly acceptable capacitors, no success was obtained in printingcommercially satisfactory resistors on any uncoated paper. The papersfound to produce the best resistive films (with the inks hereinafterdescribed) were commercially available papers of the type generallycalled clay-coated," having a fine-pared smooth printing surfaceoverlying the more coarsely porous body. Even within this group, whichare sufficiently similar so that quantitative information on relevantstructural differences is not available, there were observed differencesin merit of the circuits produced, even though these differences wererelatively small as compared with the differences between this group asa whole and papers lying outside the porosity range wherein whollyuseable resistors are obtained.

A diversity of inks were employed in the tests. It was found that themost desirable pigments for use in resistive, conductive, and dielectricink, respectively, were to a great extent independent of the other inkconstituents present, although the optimum selection of the otherconstituents, herein collectively called the vehicle, substantiallyvaried with the type of pigments, i.e., the most desirable resistive inkwould not provide the most desirable conductive ink or dielectric ink bymere substitution of one pigment for another. Two generalclassifications of vehicles, based on drying or curing mechanism, werefound most satisfactory. These are the types known as oxidizable andheatset. In an oxidizable ink, curing of the binder is effected bypolymerization and air oxidation, either at room temperature or, forsomewhat greater speed, in a suitable oven. Common components of thebinder of such an ink vehicle are certain alkyds, vegetable oils,hydrocarbons and linseed oil. The type of ink known as heatset" normallyemploys as the binder a suitable varnish thinned to the desiredconsistency by a relatively non-volatile hydrocarbon composition offairly low flash point, and is dried rapidly by a burning-off of thehydrocarbon by flaming. As hereinafter pointed out, it is found thateach of these types of inks has its own distinctive advantages in theprinting of circuits, and it is further found that a vehicle adapted tobe cured and dried by a combination of these processes produces resultswhich partake highly of the advantages of both. In principle, it is ofcourse possible to print the various patterns employed incircuit-forming with inks of greatly different vehicle characteristics,but such an approach is practically undesirable for economicalproduction printing of circuits.

Where a slowly-dried oxidizable ink is used, the beneficial effects ofthe filtering action of the paper are generally minimized. With suchinks, the drying or curing is normally delayed until there is reachedmore or less of an equilibrium between the capillary action of the poresof the paper and the capillary action produced by the pigment itselfupon reaching a fully compacted condition. However slow dryingintroduces the necessity of substantial delay between application ofsuccessive film patterns and also complicates handling of output sheets,which cannot be directly stacked for handling until drying has proceededto the point where there will be no transfer (offset") of ink from thedrying film to the back of the succeeding sheet. Drying of a heatset inkby flame treatment can be much more rapid. However, if the flametreatment is applied shortly after the time of impression, the filteringaction of the invention may not proceed to completion and the degree ofcompaction of the pigment obtained is not quite as high. The result isthat resistors printed with heatset inks and cured by flaming tend tohave a somewhat higher deviation of resistance values from one sample tothe next than the best results obtainable with the oxidizable inks.(Note that it is not generally possible to produce an exact match of theamount of pigment deposited on a given pattern area with different typesof inks but with the same settings of other press conditions so thatcorrelation between mean resistance value and absence ofsample-to-sample variations cannot be expected in such comparisons).

In the case of oxidizable inks, papers found best for use, with properpress adjustment, show as little as less than 3 percent averagedeviation of resistance values over a large number of samples in someruns. Average percentage deviations with heatset inks were somewhathigher, but the oxidizable inks were found less satisfactory when soughtto be employed for overprinting a layer already printed, particularly inbuilding up a large number of layers, as in a capacitor constructionsuch as shown in FIG. 6. Such a construction employs a conductive film30 directly deposited on the substrate 32. Over this are deposited anumber of layers (of the order of five to ten) of dielectric films toform the overall dielectric 34, upon which is printed a furtherconductor 36 forming the upper electrode of the capacitor. In printingsuch multilayer circuit elements, the heatset inks and the oxidizableinks were found to each have certain problems regarding interactionbetween layers. The fresh application of oxidizable ink excessivelyattacked the layer already dried, while heatset overprinted layersfrequently would not adhere. These problems were satisfactorily solvedwith mixtures of the two types of inks.

Exemplary oxidizable ink formulations suitable for use with clay-coatedpapers for direct surface application (i.e., as the first film or layerdeposited) are:

Resistive ink: 17 parts carbon black (Cabot XC727R); 40 parts alkyd(LV498); 31 parts Magic Oil No. 470; parts boiled linseed oil; and twoparts dryer (337).

Conductive ink: 59 parts flake silver (Silflake 135); 10 parts alkyd(V172); 10 parts Magic Oil No. 470; 10 parts boiled linseed oil; and onepart dryer (337).

Dielectric ink: 75 parts barium titanate; eight parts alkyd (V498);eight parts alkyd (V172); seven parts boiled linseed oil; and two partsdryer (337).

Desirable compositions for heatset inks are:

Resistive ink: parts carbon black (Cabot XC727R); 78 parts varnish(El4-24A); and seven parts Magie Oil No. 400.

Conductive ink: 70 parts silver flake (Silfiake 135); parts varnish(El4-24A); and 10 parts Magic Oil No. 440.

Dielectric ink: 80 parts barium titanate; 11 parts varnish (El4-34A);five parts Magic Oil" No. 440; 3% parts alkyd (V l 72); and one-halfpartdryer (337),

For composite-type inks, mixtures of equal parts of the respectiveresistive and conductive inks above are found suitable. For thedielectric ink, a desirable formulation is: 67 parts barium titanate;19% parts varnish (El4-B34A); eight parts Magic Oil" No. 440; five partsalkyd (V172); one-fourth part dryer (337); and one-fourth part non-dryer(Eugenol).

The group of coated papers mentioned above were: Appleton LetterpressOffset; Appleton Masterful Offset; Over-print Label; Lustro Gloss OffsetEnamel Finish; Lustro Gloss, Offset Dull Finish; Lustro Gloss (regularfinish); Northwest Mountie; and Cumberland Gloss.

Satisfactory results were obtained in printing resistors, employing theresistive inks above described, with all of these papers, although insomewhat varying degree, to some extent dependent upon the type of ink.It was found that results obtained with the oxidizable ink were somewhatmore sensitive to exact characteristics of the paper than the heatsetink. The papers producing the best results with the oxidizable inkdisplayed substantially lower sampleto-sample deviations of resistancevalue than were obtainable with any paper using heatset inks, butcertain papers produced less desirable (although acceptable for manypurposes) results with oxidizable inks than any of the mentioned papersproduced with heatset inks. Resistance measurements were made on runs ofa pattern of eight rectangular resistors distributed to appear at widelyseparated portions of a letter-size sheet. For each resistor of thepattern, a large number of individual samples of each paper weremeasured as to resistance and the average percentage deviation about themean resistance value calculated. In all cases, variations of thisaverage percentage deviation were observed in some degree from oneresistor of the pattern to another. Such variations were not, however,consistent from one paper to the other. It is presently hypothesizedthat these differences result from differences of degree of uniformityat various locations on samples of the same paper which are notobservable in any other known manner. Under these circumstances, it wasof course impossible to wholly isolate the effects of type-of-papervariations from random resistance variations flowing from inadequatecompaction of the pigment particles. The effects of possible variationsin impression pressure at various portions of the printing plate wereminimized by employment of an elastomeric plate producing very lightpressure at the point of impression.

As a general indicator of relative merit of the various papers, theaverage percentage variations of the values of individual resistors werethemselves averaged for all locations on the paper. For the heatsetinks, the range of overall percentage deviations of all resistances wasfrom 5.23 percent (Lustro Gloss regular finish) to 8.41 percent(Cumberland Gloss). For the oxidizable inks, the range was somewhatbroader at both extremes, ranging from 4.03 percent (Lustro Glossregular finish) to 10.26 percent (Appleton Masterful Offset). Althoughthe order of merit of the various papers was not wholly identical withthe two types of inks, the differences in merit produced by differencesin vehicle were small.

Dependent upon the degree of precision required in any particularcircuit design, most or all of the papers in this group thus producesatisfactory printed resistors. By contrast, no comparable results wereobtained with substrates of substantially different surface structure.With uncoated papers, resistance values were unusably high, and sowidely distributed from sample to sample as to make the productionprocess useless. With an impervious substrate such as a polyethylenecoated paper, the results were similarly useless when oxidizable inkswere employed; with certain heatset inks, it was found possible to printruns of individual resistors of values reasonably clustered about arelatively high mean value, but the results obtained were poorer thanthe poorest obtained within the group of clay-coated papers, in additionto the fact that adherence of the printed films could not be madereliable.

Details of the manner of curing and drying of the films are found toproduce little effect on quality if certain basic principles areobserved. It is important that sufficient vehicle penetrate into thesubstrate to produced the desired pigment compaction on the surfacebefore the drying and curing of the binder is completed. Where lightprinting impression pressure is used, as with an elastomeric plate, suchpenetration occurs relatively slowly, and excessively rapid drying, suchas by immediate flaming of a heatset ink, or by too-rapid oven baking,should be avoided. The speed of penetration varies substantially frompaper to paper and from ink to ink, and the minimum desirable dryingtime of a resistive film for a particular ink-paper combination shouldbe experimentally determined, being generally a substantial number ofminutes to an hour or more. It should be noted that with a coated paper,the time for completion of the filtering action is not determined whollyby the porosity of the coating, but depends on a number of other factorssuch as the coating thickness and the coarseness or fineness of the bodyof the paper. With desirable carbon resistive pigments, it is believedthat the end-point of penetration is more or less automaticallyestablished by the capillary counter-force exerted by the filteredpigment residue as it reaches its compacted condition. However withcoarser pigments, it may be possible to reach a condition ofover-extraction of vehicle, in which case cohesiveness of the film willbe adversely affected.

The printed circuits so produced are desirably encapsulated or terminalsextending), preferably a multilayer laminate of plastics designed toprevent all types of substances, as well as thermal effects, fromproducing change in the film resistance. A laminate includingpolyethylene, Saran, Surlyn, and nylon was found particularly desirablefor this purpose. The encapsulation may be done by dipping, and asuitable wax applied for additional moistureproofing after curing of theencapsulant. The leads or terminals, preferably attached to the printedfilm structure by a conductive epoxy cement, are of course embedded inthe encapsulant except for their extending ends, as is conventional.

As will be observed by those skilled in the art, the basic teachings ofthe invention may be employed in manners substantially different fromthe embodiments herein described in accordance with the patent laws.Accordingly, the scope of the protection to be afforded the inventionshould be determined in accordance with the definitions thereof in theappended claims, and equivalents thereto.

What is claimed is:

1. In the method of manufacturing printed electronic circuits includingthe steps of depositing on all portions of a patterned area of thesurface of each of a succession of substantially identical substrates alayer of an ink comprising a settable liquid vehicle bearing a uniformconcentration of solid pigment particles of desired electricalcharacteristics and setting the vehicle on each substrate to form asurface film of pigment particles in a vehicle matrix covering thepredetermined area, the improvement for producing uniform electricalcharacteristics comprising the steps of depositing the ink withsubstantial excess of vehicle and filtering from the surface film onsaid area, prior to setting of the vehicle, a portion of the liquidvehicle with substantially lesser pigment concentration than saiduniform concentration to produce a solidified film on the predeterminedsurface area of substantially higher pigment concentration than isproduced by setting of the vehicle at deposition concentration. 2. Themethod of claim 1 wherein the vehicle is filtered from the surface filmby absorption in a porous body.

3. The method of claim 2 wherein the porous body is the substrate.

4. The method of claim 3 wherein the porous body is paper. 5. The methodof claim 4 wherein the paper comprises a body portion having a porouscoating on said surface of substantially smaller pore-size than the bodyportion.

6. The method of claim 5 wherein the paper is clay-coated. 7. The methodof claim 1 wherein the pigment particles are carbon and the printedelements are resistors.

2. The method of claim 1 wherein the vehicle is filtered from thesurface film by absorption in a porous body.
 3. The method of claim 2wherein the porous body is the substrate.
 4. The method of claim 3wherein the porous body is paper.
 5. The method of claim 4 wherein thepaper comprises a body portion having a porous coating on said surfaceof substantially smaller pore-size than the body portion.
 6. The methodof claim 5 wherein the paper is clay-coated.
 7. The method of claim 1wherein the pigment particles are carbon and the printed elements areresistors.